It is well known that cold working a surface of a metallic part will cause a residual compressive stress layer to form. The residual compressive stress layer improves the part's durability during use by suppressing tensile stress formation. In other words, forces acting on the part must first overcome the compressive stresses in the surface before destructive tensile stresses form.
Shot peening is well known in the art of surface treatment. Shot peening is a method of cold working metallic parts to form residual compressive stresses. The shot peening process consists of blasting a part with a stream of shot. As is known in the art, when individual shot strike a surface with sufficient kinetic energy, they form depressions. The depressions form due to plastic deformation. Therefore, blasting the part with many individual shot form a plastically deformed surface layer. A residual compressive stress layer accompanies the plastic deformation. In addition, the plastically deformed surface layer has a topography that is related to the shot peening parameters.
Shot peening, however, is difficult to control and therefore provides inconsistent results. Shot peening is difficult to control simply because the shot peening process has many process parameters. For example, parameters may include shot velocity, shot impingement angle, and peening time. The shot peening process must control these factors because they influence formation of the treated surface topography and the compressive stress layer. Other parameters that affect part performance include the uniformity of the surface coverage.
The shot peening process must completely cover a treatment area for that area to benefit from the surface compression formed. Even a small untreated surface in the treatment area will operate as a “weak link” during service. Unfortunately, blasting tens of thousands of shot at the working surface does not, by itself, guarantee complete coverage. To complicate the shot peening process even more, the shot breaks down during use.
As the shot breaks down, it becomes more difficult to control. Broken shot lose their ability to form depressions at a given velocity and impact angle. Shot breakdown may include normal wear and tear, and fracture. Thus, shot breakdown lessens the repeatability of the treatment area topography and residual compressive stress layer. So, in addition to control parameters, the shot peening process must have a purging system to rid itself of poorly performing shot to maintain repeatability.
The problems with shot peening do not end with process control, surface coverage, and shot breakdown. Shot peening may damage the part. In particular, shot peening may cause sharp topography to form. The sharp topography generally occurs between depressions and are known as stress concentration points. Stress concentration points magnify the tensile stresses that develop during use. So, rather than enhancing the part's performance, shot peening may lead to premature part failure.
In addition to the problems previously stated, shot peening is limited in other ways. For example, the deflected shot should be directed away from the incoming stream. So, the shape of the part is limited to those which allow the deflected shot stream to escape. Also, any surface contamination on the part may interfere with shot impact, thus lessening the effects of the shot peening process. In addition, using shot peening to treat small target areas without affecting surrounding areas is problematic. Localized shot peening treatment requires some sort of shield or mechanism which deflects the shot stream away from areas that should not be peened.
Therefore, what is needed in the art is a method for surface treatment of parts that does not require control and processing of individual shot, that is easily directed to treat specific areas of the part, that provides control of the magnitude of the deformation, that provides control of the shape of the plastic deformation, and that provides repeatable surface treatment of delicate components. Furthermore, what is needed is a method for surface treatment that provides smooth and continuous transitions between individual deformation areas, such that formation of stress concentrations is reduced or eliminated.